On Aug 25, 2005, at 4:22 PM, A.J. Farmer (AJ3U) wrote:
> Here is a very interesting site I stumbled upon:
>>http://www.mountainlake.k12.mn.us/ham/aprs/>There is a serious problem with using the APRS Internet System data
for this purpose. The APRS IS was designed to move the data payloads,
not the path information. Duplicate packets are filtered, which make
ANY interpretation tricky, but in my opinion this site makes claims
the data cannot support.
Just as an example, the page you mention
http://www.mountainlake.k12.mn.us/ham/aprs/path.cgi?call=AJ3U-5
is not really "Stations Received By AJ3U-5". Rather, it is those
stations sending packets received by AJ3U-5 which got to this web
site before those received via other IGates. There may be many others
received in this time frame that do not appear on the list. Many, if
not most, interesting propagation events will be lost because of this
filtering. Path modifications by digipeaters may place stations here
not actually heard, though in the example I do not see any.
The site calculates hop distances between callsigns listed in the
path...however, unless everyone runs trace, callsigns are not added
or are dropped from the path in routine digipeater operation, so a
packet may have traveled through multiple digis between the two
identifiable endpoints.
The site depends on manual removal of HF stations from the analysis.
Manual filtering will miss some HF stations containing bad data, and
filter some good data. Stations transmitting on both HF an VHF are
particularly problematic.
My analysis of the problems with this interpretation of the data is
based on sketchy details on the web site, and some of the points I
make may be wrong in a specific detail, there is no question that the
raw data on the APRS IS simply does not contain the information
needed to produce the output this web site contains. The charts are
pretty though!
If you want to see propagation studies done right with modified APRS
tools, see
http://w2ev.rochesterny.org/PropNET/
Ev has created a separate internet hub without filtering designed
explicitly for propagation studies. Furthermore, the system primarily
uses PSK31, resulting in a greater sensitivity to propagation events.
It supports many bands, though almost all activity is 10 meters at
this time.
findU has separate support for PropNET, here are the paths observed
in the last day
http://propnet.findu.com/catch.cgi?last=24
There are animated GIFs for the last day and last week created each
night
http://propnet.findu.com/yesterday.gifhttp://propnet.findu.com/lastweek.gif
PropNET was a lot more work than the harvesting APRS IS data, and
because effort is required from each of the participating stations,
fewer people participate and PropNET produces much less data, but the
data it does returns has real meaning. I'd encourage anyone
interested in propagation to join Ev's effort, all he needs is more
stations and this will become a serious tool.
Steve K4HG